The use of frequency modulation techniques of the GFSK type, like that employed in the Digital European Cordless Telecommunications (DECT) standard, make it advisable to use frequency demodulators based, for example, on discriminators; in this way it is possible to recover the data by means of a simple threshold detector in order to decide between the two logic states, 0 or 1.
In certain environments where these systems are applied, this type of receiver is sufficient because the maximum spread of the signal at the receiver is considerably less than one symbol and, consequently, the intersymbol interference produced in the channel is hardly appreciable. However in applications where radio coverage is greater, the latter is not true. In such situations it becomes advisable to make use of equalisers that remedy this signal-degrading effect resulting in the error probability of recovered symbols being notably enhanced.
For this reason, use is made of an equaliser of the type illustrated in the article "Adaptive equalisation for DECT systems operating in low time-dispersive channels" by J. Fuhl and G. Schultes, published in the magazine Electronics Letters of Nov. 25, 1993, vol. 29, No.24, pages 2076 and 2077.
In this article use is made of an equaliser of the well known type DFE (Decision Feedback Equaliser). In it, the training sequence employed is the 16-bit burst synchronisation as defined by the DECT. The training sequence is stored in a ROM and applied to the equaliser at the moment when the 16 bits appear at the receiver input, instead of the actually received signal. This prevents the possible errors produced in the detector, when the filter coefficients are still inexact, from being fed back to the equaliser and, therefore, a faster convergence occurs.
Nevertheless, when the demodulator is of the non-linear type like, for example, a frequency discriminator, even under ideal conditions of propagation, the demodulated signal can be considerably distorted due to the action of the demodulator. Under these conditions, adequate convergence does not occur, since no allowance is made for the non-linear process to which the received signals are subjected.